1. Field of the Invention
This invention relates to polar thermoplastic resins having improved gas barrier properties and more particularly to a method for preparing polar thermoplastic resins filled with mica flakes having improved gas barrier properties.
2. The Prior Art
Heretofore, polar thermoplastic resins such as polyethylene terephthalate have been used as packaging materials for carbonated beverages, pharmaceutical products, cosmetic articles and other products, but because of their relatively high permeability to gases such as oxygen, their use has not been extended to packaging oxygen-sensitive food products to be stored at unrefrigerated or ambient temperatures. For example, attempts to use polyethylene terephthalate in the packaging of oxygen sensitive foodstuffs have encountered the main disadvantage that because such resin materials have relatively high permeability to oxygen, the permeation of oxygen into the package or container causes a discoloration and a depreciation in the taste qualities of the packaged foodstuff which is undesirable.
The art has devised a number of methods to reduce the gas premeability of synthetic thermoplastic resins, and included in these methods is the incorporation in the resin of a mineral filler such as mica or glass flake, e.g., U.S. Pat. Nos. 3,463,350 and 4,031,610. Although the incorporation of such fillers in resins such as polyethylene terephthalate does materially increase the gas barrier properties of the resin, the composite material is still deficient in the gas barrier properties required for the packaging of oxygen-sensitive foodstuffs. For example, it has been determined that containers fabricated from thermoplastic resins generally require an oxygen leak rate of about 0.05 cc/100 in.sup.2 /day @ 73.degree. F. or less in order to be considered for the packaging of oxygen-sensitive foods, i.e., foods such as ham which require a shelf-life of about 18-24 months at ambient, non-refrigerated conditions. Containers fabricated from thermoplastic resins such as polyethylene, polypropylene or polyethylene terephthalate filled with mineral fillers such as mica, generally have an oxygen leak rate in the order of 0.1-0.2 cc/100 in.sup.2 /day @ 73.degree. F. which is far in excess of the 0.05 cc/100 in.sup.2 /day @ 73.degree. F. leak rate considered a maximum for food containers. Thus, although the art has attempted to increase the gas barrier properties of thermoplastic resins by using mineral fillers such as mica, the gas barrier requirements for most oxygen-sensitive foodstuffs have still not been effectively met by containers fabricated from these filled resins.